MasterSonic has released a Metal Slug homebrew project. According to a Google translated copy of the release page, the game features 15 levels, custom character creation, and more. My source link has reported the game as being a verses homebrew. I have not personally tested the file yet, so please report on your findings!

Downloaded, About to play it, Will post my "review"
EDIT: runs fine on my R4, Not MUCH to do, but its good for a hombrew, Appears to be quite hard for two people to play though around a small hand held.
Some graphics arnt very good, like the knifing, Atm the time went into negetives and hp on player 2 is screwed up.

Overall, good for its first release but a few updates would be in order i say XD